1. Field of the Invention
The present invention relates to an image display system that modulates illumination light from a light source and projects and displays desired images. More particularly, this invention relates to an image display system implemented with a light source controlled to project light of variable intensities to further increase the number of gray scales for improving the quality of image display.
2. Description of the Related Arts
After the dominance of CRT technology in the display industry over the past 100 years, the Flat Panel Display (hereafter FPD) and Projection Display technologies are now gaining popularity because of a smaller form-factor of the display control system while enabled to project and display image of greater size onto a bigger display screen. Among several types of projection display systems, projection display systems using micro-display are gaining consumers' recognition because of high performance of picture quality as well as lower cost than the display systems implemented with FPDs. There are two types of micro-display technologies implemented in the projection display systems now made available in the market. The first type of display system is the micro-LCD (Liquid Crystal Display) system and the other type display system is the display system that implements the micromirror technology. Because a micromirror device uses un-polarized light, a micromirror device has an advantage that the display image projected from the micromirror device has a higher brightness over the display image projected from a micro-LCD system, which uses polarized light.
Even though there are significant advances of the technologies in implementing an electromechanical mirror device as a spatial light modulator (SLM) in recent years, there are still limitations and difficulties when it is employed to provide a high quality image. Specifically, when the images are digitally controlled, the image quality is adversely affected due to the fact that the images are not displayed with sufficient number of gray scales.
An electromechanical mirror device is drawing a considerable interest as a spatial light modulator (SLM). The electromechanical mirror device includes “a mirror array” that has a large number of mirror elements. In general, the mirror elements from 60,000 to several millions are arranged on a surface of a substrate in an electromechanical mirror device. Referring to FIG. 1A for an image display system 1 including a screen 2 is disclosed in a reference U.S. Pat. No. 5,214,420. A light source 10 is used for generating light energy for illuminating the screen 2. The generated light 9 is further collimated and directed toward a lens 12 by a mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate light 9 into a column of light 8. A spatial light modulator (SLM) 15 is controlled on the basis of data input by a computer 19 via a bus 18 and selectively redirects the portions of light from a path 7 toward an enlarger lens 5 and onto screen 2. The SLM 15 has a mirror array includes switchable reflective elements 17, 27, 37, and 47 each comprising a mirror 33 connected by a hinge 30 and supported on a surface 16 of a substrate in the electromechanical mirror device as shown in FIG. 1B. When the element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along the path 4 to impinge on the screen 2 so as to form an illuminated pixel 3. When the element 17 is in another position, the light is redirected away from the display screen 2 and hence the pixel 3 is dark.
Most of the conventional image display devices such as the devices disclosed in U.S. Pat. No. 5,214,420 are implemented with a dual-state mirror control that controls the mirrors to operate at a state of either ON or OFF. The quality of an image display is limited due to the limited number of gray scales. Specifically, in a conventional control circuit that applies a PWM (Pulse Width Modulation), the quality of the image is limited by the LSB (least significant bit) or the least pulse width as control related to the ON or OFF state. Since the mirror is controlled to operate in an either ON or OFF state, the conventional image display apparatuses have no way to provide a pulse width to control the mirror that is shorter than the control duration allowable according to the LSB. The least quantity of light, which determines the least amount of adjustable brightness for adjusting the gray scale, is the light reflected during the time duration according to the least pulse width. The limited gray scale due to the LSB limitation leads to a degradation of the quality of the display image.
Specifically, FIG. 1C shows an exemplary control circuit for controlling a mirror element according to the disclosures made in U.S. Pat. No. 5,285,407. The control circuit includes a memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; while transistors M6, M8, and M9 are n-channel transistors. The capacitances C1 and C2 represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a, which is based of a Static Random Access Switch Memory (SRAM) design. The transistor M9 connected to a Row-line receives a DATA signal via a Bit-line. The memory cell 32 written data is accessed when the transistor M9, which has received the ROW signal on a Word-line is turned on. The latch 32a includes two cross-coupled inverters, i.e., M5/M6 and M7/M8, which permit two stable states, that is, a state 1 is Node A high and Node B low, and a state 2 is Node A low and Node B high. The control circuit as illustrated in FIG. 1C controls the micromirrors to switch between two states and the control circuit drives the mirror to oscillate to either an ON or OFF deflected angle (or position) as shown in FIG. 1A.
The minimum quantity of light controllable to reflect from each mirror element for image display, i.e., the resolution of gray scale of image display for a digitally controlled image display apparatus, is determined by the least length of time that the mirror is controllable to hold at the ON position. The length of time that each mirror is controlled to hold at an ON position is in turn controlled by multiple bit words. FIG. 1D shows the “binary time periods” in the case of controlling SLM by four-bit words. As shown in FIG. 1D, the time periods have relative values of 1, 2, 4, and 8 that in turn determine the relative quantity of light of each of the four bits, where the “1” is least significant bit (LSB) and the “8” is the most significant bit. According to the PWM control mechanism, the minimum quantity of light that determines the resolution of the gray scale is a brightness controlled by using the “least significant bit” for holding the mirror at an ON position during a shortest controllable length of time.
As illustrated in FIG. 2A, when adjacent image pixels are displayed with a brightness controlled by very coarse gray scales, the adjacent pixels may be displayed with great differences of quantity of light, thus, artifacts are shown between these adjacent image pixels. That leads to the degradations of display image qualities. The degradations of image qualities are specially pronounced in the bright areas of image when there are “bigger gaps” of gray scale, i.e. quantity of light, between adjacent image pixels. For example, the bright areas are generally observed on the forehead, the sides of the nose and the upper arm in an image of human and there are artifacts shown in these bright areas when displayed with gray scales of coarse resolutions. The artifacts are caused by a technical limitation that the digitally controlled image does not obtain sufficient number of the gray scale, i.e. the levels of the quantity of light. At the bright spots of display, e.g., the forehead, the sides of the nose and the upper arm, the adjacent pixels are displayed with visible gaps of light intensities. When the levels of gray scales are increased, the image degradation is significantly reduced even with only twice more levels of gray scales as illustrated in FIG. 2B.
Therefore, when the mirrors are controlled to operate at either ON or OFF position then the adjustable quantity of light of a displayed image is controlled by the length of time each mirror is held at the ON position. In order to increase the number of the levels of the controllable quantity of light, the switching speed of the ON and OFF positions for the mirror must be increased. A higher number of bits is therefore required to control the ON and OFF states of the micromirrors. However, when the switching speed of the mirror deflection is increased, a stronger hinge for supporting the mirror is necessary to sustain a required number of switches of the ON and OFF positions for the mirror deflection. Furthermore, in order to drive the mirrors provided strengthened hinge toward the ON or OFF positions, it becomes necessary to apply a higher voltage to the electrode. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The mirrors produced by applying the CMOS technologies probably is not appropriate for operating the mirror at such a high range of voltages, and therefore the DMOS mirror devices may be required. In order to achieve a control of higher number of the gray scales, a more complicated production process and larger device areas are required to produce the DMOS mirror. Conventional mirror controls are therefore faced with a technical problem that higher level of gray scales and can only be achieved by operating the micromirrors at a range of higher voltage to maintain the benefits of manufacturing a smaller image display apparatus.
There are many patents related to the control of quantity of light. These Patents include U.S. Pat. Nos. 5,589,852, 5,617,243, 5,668,611, 5,706,061, 5,903,323, 6,232,963, 6,262,829, 6,592,227, 6,648,476, 6,819,064, and 6,975,366. There are further patents and patent applications related to different sorts of light sources. These Patents include U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. Also, The U.S. Pat. No. 6,746,123 has disclosed particular polarized light sources for preventing the loss of light. However, these patents or patent applications do not provide an effective solution to attain a sufficient number of the gray scale in the digitally controlled image display system.
Furthermore, there are many patents related to a spatial light modulation that includes the U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions do not provide a direct solution for a person skilled in the art to overcome the above-discussed limitations and difficulties. Therefore, a need still exists in the art of image display systems applying digital control of a micromirror array as a spatial light modulator to provide new and improved systems such that the above-discussed difficulties can be resolved. The major difficulty that hinders the increase of the gray scales of image display is caused by the ON-OFF control scheme of the conventional systems that limits the minimum ON time to adjust the brightness of a display image. The minimum ON time determines the height of the steps of gray scale in FIG. 2. There is no way to provide the adjustable brightness that is lower than the step by controlling the micromirrors. In order to overcome the problems of the degradation of picture quality it is necessary to increase the level of adjustable brightness with adjustable brightness lower than the step shown in FIG. 2.
There is an increasing demand for an image display system to display image with higher image quality. One of the determining factors for displaying an image with improved image quality is to increase the pixel resolution such as in a high definition (HD) display system a HD-level (1920×1080) high resolution is gaining popularity. However, another important factor for improving the image quality is to increase the level of gray scales and as of now most of image display systems still use an 8-bit display mode (8 bits for each of RGB colors, for example). In order to improve the image display quality, it is foreseeable that in the near future, it is desirable and necessary that an image display system uses a greater number of gray scales, for example, gray scales controlled by digital word 10 bits or more.
A micromirror device is implemented as a display system for wide screen image display configured either as a front projector or a rear-projection TV. A micro-mirror device as now implemented in the display systems deflects the illumination light in two directions, ON and OFF. When PWM control is used to modulate illumination light (hereinafter simply referred to as PWM modulation), each of gray scales of images or videos are expressed by the sum of the amounts of reflected light during ON time (time during which illumination light is deflected in the ON direction) in one frame period. To express 8-bit gray scales (256 gray scales), the micro-mirror device uses PWM modulation in which a minimum unit (LSB) is 1/256 of one frame period. To express 10-bit gray scales (1024 gray scales), the micro-mirror device uses PWM modulation in which the LSB is ¼ of that required to express 256 gray scales. In this case, the micro-mirror device performs ON/OFF control of each mirror element at the speed four times the speed required to express 256 gray scales. To operate a mirror element at a faster speed, it is necessary to tilt the mirror with a stronger spring. To tilt the mirror with a stronger spring, it is necessary to control the mirror element at a higher voltage. Furthermore, the number of repetitive operations of the mirror element increases. An increased number of mirror element motions reduce the lifetime of the spring that tilts the mirror. Therefore, a need still exists to further improve the image display systems such that the above discussed difficulties and limitations can be resolved.